1. Field
Embodiments of the disclosure relate generally to the field of propulsion systems for aircraft and more particularly to embodiments for a counter rotating fan with a variable blade row spacing.
2. Background
The escalating cost of aviation jet fuel, enactment of and anticipated growth in carbon-related taxation regulations has created an enthusiastic industry-wide resurgence into Prop-fan or Open-Fan technology, as demand for travel continues to climb with sustained pressure to minimize fare increases. Concurrently; due to increased travel the legal noise limits imposed by the United States Federal Aviation Administration (FAA) and the International (ICAO) agencies for engine and aircraft certification have become more stringent. In many countries, local aviation authorities have imposed a combination of fees, curfews, and quotas aimed to offset the growth of noise exposure and costs associated with abatement, including sound-proofing homes. For enforcement, numerous airports have installed microphones in noise sensitive communities that currently force operators to sacrifice payload and or range to avoid violation of these local noise policies. Furthermore, it is anticipated that local air-quality or other carbon related environmental costs could be imposed.
Additionally, evolving demand for expanded point-to-point service beyond traditional narrow-body markets and operational flexibility drives designers to further emphasize fuel efficiency relative to other tradeoffs.
Improved fuel consumption may be obtained by reducing cruise speed however this may increase flight time and be undesirable to passengers, the net fuel benefit may be fairly small, challenges with air traffic integration may be created, the number of revenue flights in a given day might be reduced, and this approach may actually result in an increase of other airline operating costs. In order to achieve the cruise speeds of today's jet powered aircraft (˜Mach 0.8) counter rotation open fan (CROF) systems are needed, since single rotation/stage turboprops are practically limited to cruise speeds of roughly Mach 0.7 due to insufficient specific thrust.
Counter Rotating Open Fans have complex noise sources that single rotation turboprops do not have; specifically propeller wake interaction and tip vortex interaction noise. Both of these noise sources may result in external environmental noise which affects airport communities, cabin noise which affects passenger comfort, and airplane structural sonic fatigue.
A highly complex trade between these noise sources and net propulsive efficiency exists for various design approaches. Wake interaction noise, which is undesirable, and propulsive efficiency, which is desired, tend to decrease with greater spacing between the fans. However, vortex interaction noise, which is undesirable, may actually increase with spacing depending on free stream Mach number, local flow effects, angle of attack and downstream propeller row diameter due to the stream tube contraction after the first row of blades.
Vortex interaction avoidance is typically of highest priority to designers, however the only means in present designs to accomplish this is to “crop” or reduce the diameter of the aft or down-stream rotor. This however may carry a performance penalty as the aerodynamic efficiency may be compromised in the same fashion as a fixed wing due to loss of span/aspect ratio. A key challenge for the designer is that vortex interaction is affected by several factors. The strength of the vortex is influenced mostly by blade tip loading and the path of the tip vortex is greatly affected by free-stream momentum and angle of attack. With lower cruise speeds the tip vortex collapses toward the root of the aft/downstream rotor interacting with it and causing vortex interaction noise. Because of this designers of CROF engines in the prior art typically have chosen an aggressive degree of spacing and cropping (10% or greater) so that vortex interaction is avoided under limiting operational conditions such as, for example, the highest thrust rating and climb trajectory with maximum vortex plume contraction. This may result in the airplane reduced performance for all operational conditions.